1) Field of the Invention
This invention relates to a process for producing sialic acids-containing lactose from sialic acids containing milk-derived liquid such as whey or skim milk.
Owing to the inclusion of physiologically-active sialic acids, sialic acid-containing lactose are useful as an effective ingredient for foods, drugs, feeds, etc.
2) Description of the Prior Art
Lactose, sialic acids, ash, etc. are contained in addition to whey proteins or milk proteins in whey by-produced upon production of cheese or casein from milk as a starting material or in skim milk also by-produced upon separation and collection of milk fat from milk.
Among these components of whey or skim milk, whey proteins, milk proteins, lactose and the like have been isolated by ultrafiltration, heating, crystallization, or the like and have been used for respective applications.
Although sialic acids have been recognized to be physiologically effective for human bodies, sialic acids contained in whey or skim milk have scarcely been utilized because no low-cost industrial technique has yet been established for the isolation of sialic acids from a permeate obtained by ultrafiltration of whey or skim milk or a deproteinization solution obtained by heat treatment of whey or skim milk. Accordingly, they have heretofore been discarded along with such permeates or deproteinization solutions.
However, it has been reported recently that saccharide chains contained in a conjugated saccharide such as a glycoprotein or glycolipid play an important role for the intercellular recognition in the living body. It is now increasingly recognized that sialic acids are particularly important as essential constituents of a receptor which performs the intercellular recognition. Further, sialic acids are contained at high levels in mother's milk and are considered to have a function as an infection protective factor for infants.
In view of the above-described physiological important of sialic acids, a great advancement has been made in the development of techniques for the isolation or extract of sialic acids from milk, including those disclosed in Japanese Patent Publication No. 21234/1965 and Japanese Patent Application Laid-Open No. 184197/1984.
However, the process disclosed in Japanese Patent Publication No. 21234/1965 can isolate only a portion of sialic acids in milk and is therefore not efficient. Further, the process of Japanese Patent Application Laid-Open No. 184197/1984 is adapted to isolate sialic acids at concentrations as high as pure forms. The latter process is therefore accompanied by a problem that its steps are too complex to use it in industrial production.
On the other hand, the following procedures are generally used for the isolation of lactose from whey.
Crude lactose having a lactose content of 85-90% is obtained by conducting a series of steps, which comprise (i) lime treatment and heating of whey, (ii) removal of proteins thus precipitated, (iii) concentration, (iv) cooling and crystallization, (v) centrifugation, and (vi) washing and drying of crystals. Further, purified lactose having a lactose content as high as 95% or even higher is also obtained by proceeding with a series of additional steps, which comprise (vii) dissolution, (viii) discoloration and filtration, (ix) concentration and crystallization, (x) centrifugation, and (xi) drying of crystals.
Although several improvements on lactose production have been made to the above process as a basic process, these processes are all intended to obtain lactose alone and the isolation or recovery of sialic acids is not contemplated of at all. Most of the sialic acids in whey therefore remain in the mother liquor of the crude lactose or in the mother liquor of the purified lactose on the process of centrifugation of above (v) and (x), so that it has been impossible to isolate and recovered lactose and sialic acids together at the same time.
Similarly, skim milk is processed only to recover proteins therefrom. Thus, skim milk is subjected to ultrafiltration or to a heat treatment at a temperature as high as 90.degree. C. or even higher, which is close to the boiling point, whereby the milk proteins are isolated and recovered. Similarly to the case of whey, no attention has therefore been paid for the recovery of sialic acids.
After the recovery of proteins and crude or purified lactose from whey or skim milk, the still remaining mother liquor has heretofore been discarded in general.